The Collaborative International Dictionary of English v.0.48:

Drainage \Drain"age\, n.
   1. A draining; a gradual flowing off of any liquid; also,
      that which flows out of a drain.
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   2. The mode in which the waters of a country pass off by its
      streams and rivers.
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   3. (Engin.) The system of drains and their operation, by
      which superfluous water is removed from towns, railway
      beds, mines, and other works.
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   4. Area or district drained; as, the drainage of the Po, the
      Thames, etc. --Latham.
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   5. (Surg.) The act, process, or means of drawing off the pus
      or fluids from a wound, abscess, etc.
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   Drainage tube (Surg.), a tube introduced into a wound,
      etc., to draw off the discharges.
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